A conventional radio base station in a cellular communications system is generally located in a single location, and the distance between the baseband circuitry and the radio circuitry is relatively short, e.g., on the order of one meter. A distributed base station design, referred to as a main-remote design, splits the baseband part and the radio part of the base station. The main unit (MU) performs base band signal processing, and one or more radio remote units (RRUs) converts between baseband and radio frequencies and transmits and receives signals over one or more antennas. Each RRU serves a certain geographic area or cell. Separate, dedicated optical links connect the main unit to each of the plural radio remote units. Each optical link includes one optical fiber for carrying digital information downlink from the main unit to the RRU and another optical fiber for carrying digital information uplink from the RRU to the main unit.
Some mobile communication standards, e.g., the code division multiple access (CDMA) cellular system, permit a UE to communicate with two or more RRUs of the same base station using “softer handover.” In softer handover, two or more RRUs simultaneously transmit the same information to the UE and receive the same information from the UE. The simultaneously transmitted signals must be processed to generate a single signal. Some radio standards require that in the downlink direction, the signals simultaneously transmitted to the UE from different antennas be aligned with a timing reference at the antennas. That alignment makes combining those different signals easier on the receiver. In the uplink direction, the main unit base band functionality includes a rake receiver which combines the “same” signals received from the UE via the RRUs and generates a single signal. Because of differing path lengths to each RRU, these signal components received at the main unit base band functionality from different radio remote units are not time and phase aligned to each other. Although a rake receiver can combine out-of-phase signals from different signal paths, a less complicated and less expensive rake receiver may be used if the phase/delay differences between different signal paths are kept small.
In a main-remote radio base station, a significant phase or timing difference may be attributed to the different lengths of the optical fibers coupling different RRUs to the main unit as compared to a conventional base station. Different optical link delays are more problematic as the distance between the remote unit 16 and the main unit increases, e.g., 10 kilometers. In addition, such delays are not constant and may vary depending on temperature and other factors. Without compensation, the different optical fiber lengths to the remote units result in a time/phase shift of the signals sent out from the antennas connected to the radio remote units. They also lead to larger time/phase shifts between the UE signal components received via different radio remote units. These time/phase shifts may be difficult for conventional receivers in the UE and in the base station to handle. A similar problem exists in a hybrid base station that incorporates both conventional near radio units and remote radio units. The near radio units, which do not have any optical link delays, are not synchronized with the remote radio units that do have link delays. One optical link delay/synchronization solution is presented in commonly-assigned, U.S. patent application Ser. No. 10/252,827, filed on Sep. 23, 2002, entitled “Synchronizing Radio Units In A Main-Remote Radio Base Station And In A Hybrid Radio Base Station,” the disclosure of which is incorporated herein by reference.
Another challenge in main-remote configurations is how best to connect the main and remote units. Separate optical fibers with associated separate lasers and light detectors may be used to link the main unit with each remote unit. Each RRU communicates with the main unit using its own dedicated optical fiber loop. But the amount of fiber required is significant—as is the cost for separate main unit—RRU fiber loops. The cost of the laser and detector equipment associated with each fiber pair in the main unit is also significant. And in some deployment scenarios may require cascading several remote units, e.g., along a highway, in a tunnel, or along an existing fiber infrastructure, e.g., a metro ring. So it would be desirable to connect the main unit with each remote unit using a single fiber.
FIG. 1A shows an example of a main-remote base station system at reference numeral 10 where the main unit and RRU are connected in cascade using a single fiber. The main unit 12 includes radio base station baseband (BB) functionality 14. An optical fiber divided into four links L1–L4 connects the main and remote units in a loop. A first optical link L1 couples the main unit 12 to a first radio remote unit 16a. A second optical link L2 couples the main unit 12 to a second radio remote unit 16b. A third optical link L3 couples the main unit 12 to a third radio remote unit 16c. A fourth optical link L4 couples the third radio remote unit 16c to the main unit 12. Of course, additional radio remote units could be coupled to the main unit 12. A mobile radio user equipment (UE) 18 and one or more of the radio remote units 16a–16c communicate over a radio interface.
In cascade and ring topologies, where the units are connected in series, wavelength division multiplexing (WDM) may be used to reduce the amount of fiber used and the laser/detector equipment. Each remote unit is assigned its own, corresponding laser wavelength. The different wavelength communications for all of the remote units are multiplexed onto a single fiber. One downlink fiber is used for traffic from the main unit to all the remote units, and one uplink fiber is used for traffic from the remote units to the main unit making up a single fiber loop. An optical add/drop multiplexer (OADM) is located inside or near each remote unit. The OADM adds or drops only the unique wavelength related to that particular remote unit to the fiber. A drawback with this approach is the expense of WDM technology including lasers, filters, and OADMs. Another is logistical overhead to keep track of different wavelength dependent devices.
It is an object of the present invention to provide a cost effective optical fiber configuration to couple a base station main unit and plural base station remote units.
It is an object of the invention to provide such a cost effective optical fiber configuration that requires a single optical downlink fiber path and a single optical uplink fiber path, (together forming a single optical fiber path), to carry information between the main unit and the remote units.
It is an object of the invention to provide one or more single optical fiber pair configurations that avoid some or all of the expense of WDM technology.
It is an object of the invention to provide such a cost effective fiber configuration that avoids some or all of the logistical WDM overhead to keep track of different wavelength dependent devices.
It is a further object to compensate for time delay differences associated with different remote units coupled in a cascade, loop, or ring optical fiber configuration where the units are connected in series.
The present invention solves the problems identified above and satisfies the stated and other objects. A main-remote radio base station system includes plural remote radio units. Optical fiber costs are significantly reduced using a single optical fiber loop (one downlink fiber path and one uplink fiber path) to communicate information between the main unit and the remote units in a cascade, loop, or ring configuration. Example configurations are described below.
Information from the main unit is sent over a first fiber in the pair to the remote units so that the same information is transmitted by the remote units at substantially the same time. Assuming there are N remote units, N being a positive non-zero integer), the main unit sends out the information over the first fiber at N times the rate at which data is to be received at each remote unit. The main unit receives the same information from each of the remote units over the second fiber at substantially the same time. A data distribution approach over a single fiber loop avoids the expense of WDM technology including lasers, filters, and OADMs as well as the logistical overhead needed to keep track of different wavelength dependent devices.
The main unit combines N words of data, one word corresponding to each of the N remote units, into a frame and transmits the frame on the fiber. From the received frame, each remote unit removes its corresponding data word, includes an uplink word in the removed word's place, and passes the frame to the next remote unit. Because one fiber loop carries all of the information for each of the N remote units, the data rate is N times the data rate that would be used if each remote unit was coupled to the main unit with its own fiber loop.
Delay associated with each remote unit is compensated for by advancing a time when information is sent to each remote unit. A timing compensator for each remote unit compensates for any associated delay. Information is sent in advance of the time when it would otherwise be sent without that delay, i.e., in a conventional base station. As a result, the information is received at each of the remote radio units at substantially the same time as in conventional radio base stations with only near radio units, despite the different delays associated with each remote radio unit. The advanced-in-time transmission together with equalization for the uplink direction also ensures that a response sent by each of the remote radio units is received in the main unit at substantially the same time, despite the different delays associated with each remote radio unit.
Based on the delays received for each remote unit, the timing compensation controller selects a maximum delay. In an example embodiment, that delay corresponds to the delay associated with the remote radio unit farthest from the main unit. An advanced transmit time is determined for each remote radio unit based on the maximum link delay. In a specific example embodiment, the transmission time for digital timing and data signals is advanced by twice the maximum link delay.
The main digital interface unit includes for each remote radio unit a transmit buffer and a receive buffer. The timing compensation controller sets the transmit buffering time that the data signal is stored in the transmit buffer before the data signal is sent on the one or more digital data channels. A responsive data signal from the remote digital interface unit is stored in the receive buffer for a receive buffering time. The sum of the transmit buffering time or receive buffering time and the delay for the remote unit equals the maximum delay. Delay differences associated with distance differences on the order of meters up to 100 kilometers or more can be compensated.
The invention may also be employed in a hybrid radio base station that includes both near/conventional and remote radio units.
The present invention provides a cost effective optical fiber configuration to couple a base station main unit and plural base station remote units. Only a single optical fiber loop is needed to carry information between the main unit and the remote units. The configuration avoids the expense and drawbacks if WDM technology were used in a single fiber loop configuration. Lasers, filters, and optical add/drop multiplexers (OADMs) for each RRU are not needed thereby eliminating costs necessary for a WDM fiber loop configuration. The logistical WDM overhead required to keep track of different wavelength dependent devices is also avoided. In addition to cost savings, the invention compensates for time delay differences associated with different remote units coupled in series by a single fiber to ensure synchronization.